Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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SHAPED TRILOBAL PARTICLES
FIEZD OF THE INVENTION
The present invention relates to formed particles
having a specific shape which particles may be employed
in a wide variety of duties, catalytic or non-catalytic.
They can be suitably applied to prevent or substantially
reduce fouling of catalyst beds exposed to charges
containing fouling material, thereby reducing increases
in pressure drop. They can also be applied in
hydroprocessing, e.g. in hydrodesulphurisation and
hydrocracking, e.g. to produce middle distillates from
paraffinic material obtained via a Fischer-Tropsch
process.
BACKGROUND OF THE INVENTION
In the past a tremendous amount of work has been
devoted to the development of particles, in particular
catalytically active particles, for many different
processes. There has also been a considerable effort to
try to understand the advantages and sometimes
disadvantages of effects of shape when deviating from
, conventional shapes such as pellets, rods, spheres and
cylinders for use in catalytic as well as non-catalytic
duties.
Examples of further well-known shapes are rings,
cloverleafs, dumbells and C-shaped particles.
Considerable efforts have been devoted to the so-called
~~polylobal"-shaped particles. Many commercial catalysts
are available in TL (Trilobe) or Qh (Quadrulobe) form.
They serve as alternatives to the conventional
cylindrical shape and often provide advantages because of
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their increased surface-to-volume ratio which enables the
exposure of more catalytic sites thus providing more
active catalysts.
An example of a study directed to effects of
different shapes on catalytic performance can be found in
the article by I. Naka and A. de Bruijn (J. Japan Petrol.
Inst., Vol. 23, No. 4, 1980, pages 268-273), entitled
"Hydrodesulphurisation Activity of Catalysts with Non-
Cylindrical Shape". In this article experiments have been
described in which non-cylindrical extrudates with cross-
sections of symmetrical quadrulobes, asymmetrical
quadrulobes and trilobes as well as cylindrical
extrudates with nominal diameters of 1/32, 1/16 and
1/12 inch were tested in a small bench scale unit on
their hydrodesulphurisation activity (12 owt Mo03 and
4 owt Co0 on gamma alumina). It is concluded in this
article that the HDS activity is strongly correlated with
the geometrical volume-to-surface ratio of the catalyst
particles but independent of catalyst shape.
In EP-A-220933, published in 1987, it is described
that the shape of quadrulobe-type catalysts is important,
in particular with respect to a phenomenon known as
pressure drop. From the experimental evidence provided it
appears that asymmetric quadrulobes suffer less from
pressure drop than the closely related symmetrical
quadrulobes. The asymmetrically shaped particles are
described in EP-A-220933 by way of each pair of
protrusions being separated by a channel which is
narrower than the protrusions to prevent entry thereinto
by the protrusions of an adjacent particle. It is taught
in EP-A-220933 that the shape of the particles prevents
them from "packing" in a bed causing the overall bulk
density of the catalyst bed to be low.
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Since many of the findings in the art are conflicting
and pressure drop problems continue to be in existence,
especially when surface-to-volume ratios are increased by
reducing particle size, there is still considerable room
to search for alternative shapes of (optionally
catalytically active) particles which would diminish or
even prevent such problems. It has now surprisingly been
found that specifically shaped particles of the general
"trilobal" shape offer unexpected and sizeable advantages
compared with conventional "trilobal" particles, both in
catalytic and non-catalytic duty.
DETAILED DESCRIPTION OF THE INVENTION
The present invention therefore relates to an
elongate, shaped particle comprising three protrusions
each extending from and attached to a central position
aligned along the central longitudinal axis of the
particle, the cross-section of the particle occupying the
area encompassed by the outer edges of six outer circles
around a central circle minus the area occupied by three
alternating outer circles, wherein each of the six outer
circles is touching two neighbouring outer circles and
wherein three alternating outer~circles are equidistant
to the central circle, have the same diameter, and may be
attached to the central circle.
, It has been found that the particles according to the
present invention, having a larger surface-to-volume
ratio than corresponding conventional "trilobal"
particles of similar size, suffer substantially~less from
pressure drop than such corresponding conventional
"trilobal" particles. Moreover, the shape of the
particles according to the present invention allows a
certain degree of "packing" which according to the
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teaching of EP-A-220993 would be detrimental with respect
to pressure drop.
It has also been found that particles having a shape
in accordance with the present invention perform
exceptionally well when used as a grading material to
capture fouling, thereby guarding a fixed-bed reactor
against pressure drop increase. It is also believed that
catalysts based on particles in a shape according to the
present invention are capable of improved performance
when used in mass transfer or. diffusion limited reactions
in fixed-bed reactors, for instance as hydrocracking
catalysts in the hydrocracking of paraffinic materials
produced from synthesis gas via the Fischer-Tropsch
process.
The particles according to the invention are elongate
and have three protrusions, each running along the entire
length of the particle. The cross-section of the
particles can be described as the area encompassed by the
outer edges of six circles around a central circle minus
the area occupied by three alternating outer circles.
Each of the six outer circles is~touching two
neighbouring outer circles and does not overlap with the
two neighbouring outer circles. The six outer circles can
be seen as two sets of alternating outer circles, i.e.
the three alternating outer circles that are within the
cross-sectional area and the remaining three alternating
outer circles. The three alternating circles are
equidistant to the central circle, have the same
diameter, and may be attached to the central circle. The
distance to the central circle and the diameter of the
circles may be different for both sets of alternating
outer circles.
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Preferred particles according to the present
invention have a cross-section in which three alternating
circles have a diameter in the range between 0.74 and 1.3
times the diameter of the central circle. Preferably, all
six outer circles have a diameter in this range.
More preferred particles according to the present
invention are those having a cross-section in which three
alternating circles have the same diameter as the central
circle. Preferably, all six outer circles have the same
diameter as the central circle.
Most preference is given to particles having a cross-
section in which three alternating circles are touching
the central circle. Preferably, all six outer circles are
touching the central circle.
In Figure 1 a cross-sectional view of the mast
preferred particles according to the invention has been
depicted. The cross-sectional area of the particle of
Figure 1 is the area within the solid line 1. It will be
clear from this Figure (depicting the cross-section of
the preferred particles) that in the concept of six outer
circles of even size aligned around a central circle of
the same size, each outer circle touches its two
neighbour outer circles and the central circle whilst
subtraction of three alternating outer circles (dotted
line 2) provides the remaining cross-sectional area,
built up from four circles (the central circle and the
three remaining alternating outer circles) together with
the six areas (3) formed by the inclusions of the central
circle and six times two adjacent outer circles. The
nominal diameter for the preferred particles is indicated
as d nom in Figure 1.
The cross-sectional circumference of the particles
according to the present invention is such that it forms
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a smooth line, which can also be expressed as the
function describing the cross-sectional circumference
being continuously differentiable.
It will be clear that minor deviations from the shape
as defined are considered to be within the scope of the
present invention. It is known to those skilled in the
art to manufacture die-plates which tolerances can be
expected in practice when producing such die-plates.
It is possible to produce particles according to the
present invention which also contain one or more holes
along the length of the particles. For instance, the
particles can contain one or more holes in the area
formed by the central cylinder (the central circle in the
cross-section given in Figure 1) and/or one or more holes
in one or more of the alternating cylinders (the
alternating outer circles in the cross-section given in
Figure 1). The presence of one or a number of holes
causes an increase of the surface-to-volume ratio which
in principle allows exposure of more catalytic sites and,
in any case, more exposure to incoming charges which may
work advantageously from a catalytic and/or fouling
rejection point of view. Since it becomes increasingly
difficult to produce hollow particles as their size
becomes smaller, it is preferred to use massive particles
(still having their micropores) when smaller sizes are
desired for certain purposes.
It has been found that the voidage of .the particles
according to the present invention is well above 500
(voidance being defined as the volume fraction of the
open space present in a bed of particles outside the
particles present, i.e. the volume of the pores inside
the particles are not included in the voidage). The
particles used in the experiment to be described
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hereinafter had a voidage of typically 58o which is
substantially above that of the comparative "trilobal"
particle, the voidage of which amounted to just over 430.
The particles according to the present invention can
be described as having a length/diameter ratio (L/D) of
at least 2. The diameter of the particles is defined as
the distance between the tangent line that touches two
protrusions and a line parallel to this tangent line,
that touches the third protrusion. It is indicated as d
nom in Figure 1. Preferably, the particles according to
the present invention have a Z/D in the range between 2
and 5. For example, the particles used in the experiment
to be described hereinafter had a Z/D of about 2.5.
The length of the particles in accordance with the
present invention is suitably in the range between 1 and
mm, preferably in the range between 3 and 20 mm,
depending on the type of application envisaged. For use
in fouling control and in hydrodesulphurisation particles
can conveniently be used which have a diameter in the
20 range between 2 and 5 mm.
The shaped particles can be formed of any suitable
material provided it is capable of being processed
through die-plates giving them their intended shape.
Preference is given to porous materials which can be used
25 in catalytic as well as in non-catalytic applications.
Examples of suitable materials are inorganic refractory
oxides such as alumina, silica, silica-alumina, magnesia,
titania, zirconia and mixtures of two or more of such
materials. The choice of the material will normally
depend on the envisaged application. It is also possible
to use synthetic or natural zeolites, or mixtures
thereof, optionally together with one or more of the
refractory oxides referred to hereinabove, as the
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materials) to be used to form the shaped particles
according to the present invention. Good results can be
obtained with (catalytically active) particles based on
alumina, in particular with gamma-alumina, and various
forms of silica-alumina, but other materials can also be
applied satisfactorily.
In the event that the particles according to the
invention are to be used in catalytic processes, the
appropriate amounts) of catalytically active metals)
and/or metal compounds) will have to be present on the
particles, which then serve as catalyst carrier (in
addition to their capacity to abate fouling as the case
may be). Those skilled in the art know which metals)
and/or metal compounds) to apply for specific
applications and also to which extent and how to
incorporate the chosen moieties on the particles
envisaged.
When, for instance, hydrodesulphurisation of hydro-
carbonaceous feedstocks is envisaged, the shaped
particles according to the present invention will
normally contain one or more metals) of Group VI and/or
one or more non-noble metals) of Group VIII of the
Periodic Table of the Elements which are conveniently
present as oxides and/or as sulphides. When the
expression "hydrodesulphurisation" is used throughout
this specification it also includes hydrodenitrogenation
and hydrogenation as these hydrotreating processes
normally take place at the same time.
Hydrodesulphurisation conditions normally comprise a
temperature in the range between 150 and 400 degrees
centigrade, a hydrogen partial pressure up to 80 bar and
a ZHSV in the range between 1 and 20 N1 feed/1
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catalyst/hr. The H2/hydrocarbon feed ratio is suitably in
the range from 100 to 2000 N1/1.
The particles according to the present invention can
be used advantageously in guard bed duty. Guard beds are
normally applied to protect other catalytic beds
downstream of the guard bed against unwanted influences
caused by the feedstream to be processed over such
catalytic beds.
Fouling is one of the most encountered problems when
processing feedstocks through one or more catalytic beds.
The fouling observed can be caused by impurities in the
feedstock which were either present already or which may
have been formed during the process. Examples of
impurities present in the feedstock to.be treated are,
for instance, metal-containing particles and/or clay or
salt particles which had not or had only been removed
incompletely prior to processing over the appropriate
catalytic bed(s). Examples of impurities formed during
processing are, for instance, fragments of catalytic
active particles which were removed from the catalytic
beds) which in recycle operation are passed through such
catalytic beds) or coke particles formed during exposure
of the feedstock to (severe) process conditions.
Guard beds are normally placed upstream of the beds)
used in the catalytic process. One or more guard beds can
be used to absorb the impurities, thereby delaying the
occurrence of pressure drop which allows a longer on
stream time of the process envisaged. It is also possible
to provide part or all of the particles forming the guard
bed with catalytically active materials, thereby
combining guard and reaction duty. It is also possible to
incorporate catalytically active material of a different
nature than that used in the process as envisaged in the
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particles of the guard bed. For instance, materials
active in hydrotreating may be present in and/or on the
particles forming the guard beds) having the duty to
protect one or more catalyst beds used in hydrocracking
and placed downstream of the guard bed. The type and
amount of catalytically active materials present in such
guard beds are well known in the art and those skilled in
the art know how to employ them.
Specific applications for the particles according to
the present invention are as grading layers to protect
fixed-bed reactors prone to heavy (feedstock originating)
fouling which may occur in hydroconversion, in particular
in hydrodemetallisation processes, long residue hydro-
desulphurisation processes and in the processing of
thermally cracked material and to protect fixed=bed
reactors suffering from fines deposition deep in the
catalytic beds, for instance in units processing
synthetic crudes.
It has been found that the beds containing particles
according to the invention have - in a random packing - a
much higher voidage than beds containing the
corresponding conventional trilobes when packed using the
well known "sock loading" technique. The voidage obtained
when using the conventional trilobal shape amounts to
about 45o whereas use of the particles according to the
present invention produces a voidage of at least 550
which makes such particles attractive for low pressure
drop applications, for instance under conditions of
countercurrent gas-liquid flow.
The particles according to the present invention can
also be suitably applied in a process for the production
of middle distillates from synthesis gas in which heavy
paraffinic material produced from carbon monoxide and
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hydrogen is subjected to a hydrocracking process to
produce middle distillates in the presence of a catalyst
containing particles according to the present invention
which also contain one or more metals(s) and/or metal
compounds) possessing the desired catalytic activity.
The invention will now be illustrated by means of the
following non-limiting examples.
EXAMPLE 1
Two model experiments were carried out to monitor the
pressure drop under fouling conditions of catalyst
particles made up of conventional trilobes (to be
referred to hereinafter as TL) and of particles having a
shape as shown in Figure 1 (to be referred to hereinafter
as STL -"special" trilobes, having a cross-section
occupying the area inside seven circles of the same size
(the central circle attached by six outer circles of the
same size and three alternating outer circles forming
part of the cross-section) minus the three remaining
outer circles).
The TL particles had a nominal diameter of 2.5 mm, an
L/D of about 2.5, and were made of gamma alumina. A
randomly packed bed of the TL particles showed a voidage
of 430. They did not contain additional catalytic
material. The STL particles had a nominal diameter of
2.8 mm, an L/D of about 2.5, and consisted of material
normally used for DN-200 catalysts (commercially
available from Criterion Catalyst Company). A randomly
packed bed of the STL particles showed a voidage of
58.30. Both types of particles were obtained by extrusion
using an appropriate die plate.
The fouling material used in the two experiments
consisted of a mixture of crushed silica and FCC (Fluid
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Catalytic Cracking) catalyst. The composition of the
fouling material is given in Table 1 below.
TABLE 1
Size (nm) Fraction (%w/w) Type of material
1'.4-1.7 0.58 ~ silica
1.18-1.4 0.71 silica
0.6-1.18 6.60 silica
0.355-0.6 4.51 silica
0.212-0.355 4.85 silica
0.125-0.212 7.01 silica
< 0.125 75.74 FCC cat.
The experiments were carried out in a single column
containing the material to be tested. The column was
operated with cocurrent gas (air) and liquid (water) flow
at ambient temperature and pressure. Gas and liquid
superficial velocities were 100 mm/s and 4 mm/s,
respectively. Before each experiment, the packing was
properly wetted with clean water.
The experiments started by switching the liquid feed
from clean water to a slurry containing 2.94 kg.m3 of the
fouling material. This concentration is several orders of
magnitude higher than that to be expected under normal
operating conditions in order to be able to assess the
phenomenon of pressure drop within a relatively short
time. It was found that the run time for the TL particles
(before a pressure drop of 500 mBar/m was observed)
amounted to 1460 seconds whereas the use of STL particles
allowed for a run time of no less than 2260 seconds, i.e.
a 55o increase compared to the conventionally shaped
particles.
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~'YZ1MDT.L'
Two experiments were carried out to compare flooding
limits occurring when using conventional TL and particles
having a shape according to the present invention (in
this case, as shown in Figure 1). The particles used in
these experiments had the same shapes and compositions as
those described in Example 1. A randomly packed bed of
the TL particles showed a voidage of 40o and that of STL
particles showed a voidage of 550.
The experiments were carried out in a single column
operated countercurrently with n-octane and nitrogen at
ambient temperature and 2 bar absolute pressure. Care was
taken to ensure uniform gas and liquid distribution.
During the experiments, gas flow was increased at a
constant liquid flow rate and pressure drop was.~measured
across the column. The flood point is defined as the
point where the.pressure drop dependence on the gas
velocity abruptly changes from an order between one and
two to a substantial higher order.
In the experiment carried out with TL, the gas
velocity at which flooding started was determined at an
absolute pressure of 2 bar and a superficial liquid
velocity of 3 mm/s. The STL were tested at the conditions
at which the TL showed starting of flooding at 2 bara and
a liquid superficial velocity of 3 mm/s. At these
conditions, the gas velocity could be increased as much
as 3.4 times before the STL showed the onset of flooding.
The use of STL, therefore, delayed reaching of flooding
conditions substantially.
